Oxygen-containing free radicals including, for example, hydrogen peroxide, singlet oxygen, peroxynitrite, and superoxide can occur in cells as part of the normal metabolism of nutrients. These reactive oxygen species can cause oxidative damage to sensitive cellular components. Their potential to damage cells is controlled in part by antioxidants and enzymes such as catalase, selenium-dependent glutathione, superoxide dismutase, and thioredoxin hyperoxidase, which are involved in either destroying the reactive oxygen species or repairing oxidative damage (Butterfield et al., Amino Acids, 2003, 25(3-4):419-425). Thioreductases also can be involved in repair of oxidative damage by converting disulfides, formed during an episode of oxidative stress, to thiols. When redox signaling and control systems fail or are disrupted, reactive oxygen species can accumulate, causing oxidative stress (Jones, D. P., Antioxid Redox Signal, 2006, 8(9-10):1865-1879).
Oxidative damage to DNA, RNA, and/or proteins can threaten the survival of a biological system (Butterfield et al., Amino Acids, 2003, 25(3-4):419-425). High concentrations of reactive oxygen species have been implicated in varied conditions such as, for example, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS), atherosclerosis, diabetes mellitus, chronic renal failure, chronic lung disease, cancer, and many inflammatory diseases mellitus, chronic renal failure, chronic lung disease, cancer, and many inflammatory diseases (Haulica et al., Rom J Physiol, 2002, 37(1-4):15-27; Butterfield, D., Brain Res, 2004, 1000 (1-2):1-7; Dalle-Donne et al., Trends in Mol. Med., 2003, 9(4):169-176; Levine et al., Free Radic Biol Med, 2002, 32(9):790-796). One way that a protein may be irreversibly oxidized is though carbonylation (Dalle-Donne et al., Clin Chim Acta, 2003, 329(1-2):23-38).
The introduction of carbonyl groups into proteins can occur by i) cleavage of an amino acid side chain, ii) scission of the protein backbone, addition of lipid oxidation products, or iv) oxidation at a glycation site. Under severe oxidative stress, multiple carbonyl groups can be formed on a single protein.
There are many ways that carbonyl groups may be directly introduced into a protein. One way is through oxidation of a side chain of a proline, arginine, lysine, or threonine amino acid residue (Amici et al., J Biol Chem, 1989, 264(6):3341-3346). Metal-catalyzed oxidation can produce unique products such as glutamate semialdehyde or aminoadipic semialdehyde (Requena et al., PNAS, 2001, 98(1):69-74). Carbonyl groups also can be directly introduced into proteins by cleavage of the protein backbone. Another way that carbonyl groups can be directly introduced into proteins is via α-amidation or diamide pathways. Still another way is by oxidation of glutamyl or aspartyl amino acid side chains, in which case the product generated is N-acylated with a pyruvyl group (Stadtman et al., Amino Acids, 2003, 25(3-4):207-218).
There are also many ways that carbonyl groups may be indirectly introduced into a protein. One way is by addition of a carbonyl-containing side chain functional group such as, for example, 4-hydroxy-2-noneal, 2-propenal, or malondialdehyde at a cysteine, histidine, or lysine amino acid residue (Redox Proteomics, Dalle-Donne, I., Scalone, A. and Butterfield, D. eds., John Wiley & Sons, Inc., Hoboken, N.J., 2006, pp. 487-525). A second route by which carbonyl groups may be indirectly introduced into a protein is oxidation of advanced glycation end (AGE) products. This route is initiated by non-enzymatic addition of glucose to lysine residues to form a Schiff base that undergoes Amadori rearrangement. Subsequent oxidation of these glycated proteins results in the formation of carbonylated proteins (Dalle-Donne et al., Clin Chim Acta, 2003, 329 (1-2):23-38).